Properties of primary importance in a coating mix are its viscosity, solids content and gas content. Generally, the gas content is known as the air content, because the gas entrained in the coating mix is chiefly air. The information obtained from the measurements can be utilized, e.g., in the control of a coating process or preparation of a coating mix. Herein, the parameters to be controlled are, e.g., the amount of diluting solution to be added to the coating mix, the composition of the coating mix or the efficiency of the air purging apparatus used.
The properties of the coating mix are reflected in the quality of the final coat. For instance, the viscosity of the coating mix has an influence over the entire coating process. As the viscosity of the coating mix governs the flow properties of the coating in the interior of the applicator apparatus, obviously the operation of the coater itself is affected thereby. The coating mix viscosity also contributes to the thickness and final quality of the applied coat. Even relatively small changes in the coating mix composition can be seen in the coating mix viscosity and, thus, in the final coat quality. The coating mix viscosity in turn can be controlled by altering the mutual proportions and selection of the components used in the coating mix formula.
Also the gas content of the applied coating mix affects the attainable coat quality. An excessively high gas content gives rise to quality degradation, in addition to causing foaming in the coating mix containers. To avoid such problems, the machine circulations of coaters are generally complemented with such aids as centrifugal air purging units or, alternatively, the use of antifoam chemicals. For effective control of gas purging, the system should be equipped with a gas content analyzer.
In the prior art, the properties of a coating mix have been determined by first sampling the coating mix to be applied and then analyzing the samples in a laboratory. This approach is handicapped by involving such a long delay between the sampling instant and the assessment of the laboratory results that the online properties of the coating mix have already changed during the delay. When taking samples from the machine circulation of a coater station, a further difficulty has been experienced from the release of the entrained gas from the sample to the surroundings prior to the measurement of the sample. In other respects, too, this type of so-called off-line technique has not been found practical in the attempt to reduce the short-term variations of coating mix properties. Off-line measurement methods and the characteristics of coating formulas are described, e.g., in a publication authored by I. Roitto, T. Jarvensuu and J. Koskinen: "The Significance of Deaeration in the Coating Process", PTS Streicherei Symposium 97.
This publication also suggests that the density of a coating mix can be measured by an on-line type of measurement using a mass flow rate measurement instrument. If such a measurement is carried out at two different pressures selected in a suitable manner, the gas content of the coating mix can also be determined.
Furthermore, in a publication by R. Rauch, R. Sangl, H. -H. Hofer and J. Weigl: "Gase in Streichfarben--Auswirkungen auf Lauf--und Qualitatseigenschaften", PTS Streicherei Symposium 97, is described an on-line method for measuring the gas content of a coating mix. This method is based on passing a sample of the coating mix from the machine circulation of the coater via a piping into a measurement chamber having temperature and pressure sensors adapted therein. The measurement chamber is also provided with a stepper motor and a piston suited for adjusting the interior volume of the measurement chamber. As an additional feature, the interior volume of the chamber can be determined from the position of the stepper motor. After the pressure and temperature of the coating mix are measured using at least two different volumes of the measurement chamber, the gas content of the coating mix can be computed from known physical formulas. After the measurement, the measured sample can be returned back to the machine circulation and the measurement chamber can be flushed.
Also the on-line measurement of coating mix viscosity is known in the art. A plurality of different techniques for measuring viscosity have been disclosed in the prior art. In regard to a coating mix, a particularly advantageous technique of viscosity measurement is the capillary measurement method. This method is based on passing the coating mix flow via a capillary tube and then computing the viscosity from the pressure loss across the capillary tube that can be measured with the help of a differential pressure transducer, for instance.
A drawback of conventional techniques is that, for a multivariable assessment of coating mix properties, a plurality of separate measurement equipment must be connected to the machine circulation. This is a clumsy arrangement in which the measurement data and accuracy of results are difficult to control. Such a system may also require the same parameters to be measured several times in conjunction with the separate measurements. A further problem in the use of separate measurement apparatuses is that the composition of the coating mix under measurement may change from one measurement point to another. Finally, it must be noted that an on-line method for the measurement of dry solids content in a coating mix has not been disclosed in the prior art.